1. Field of the Invention
The present invention relates to a glass spraying method and apparatus for forming a glass coating on a metal surface.
2. Description of the Related Art
For example, as a roll for printing or an industrial roll used for papermaking or steelmaking, rolls made of metal have been used widely. Since the metallic rolls are brought into contact with a printing ink exhibiting a strong acidity or alkalinity, a cleaning solution, paper or metallic products, etc., the deterioration in quality of the surfaces of the metallic rolls is accelerated with time due to corrosion, etc. The rolls are, in many cases, used under hostile conditions such as high temperature and high humidity and have a problem that they are inferior in durability since the surface thereof is deteriorated rapidly due to the contamination on the roll surface.
Furthermore, a roll for conveying goods or articles or a guiding roll have preferably electrically insulating property since it has been recently used with electronic equipment, and it is preferable that contaminant hardly stick to the roll surface or can be removed easily from the surface if the contaminant sticks to the surface.
To solve the above-mentioned technical problems, a roll having a glass coating on the surface thereof has been proposed. The inventors has disclosed a roll base having a glass surface and a method for manufacturing the same in Japanese Kokai No. 4-99259 and a spray gun for a spraying apparatus useful for forming a glass coating in Japanese Patent No. 2562801. A technology to form a glass coating sprayed on the surface of a metallic roll is disclosed in Kokai No. 64-13324.
It is needless to say that controlling the temperature of a metallic base is important upon forming a glass coating by spraying a glass material onto the metallic base. According to the research of the inventors, it is important to heat the metallic base, upon spraying, to a high temperature of about 600 to 1000.degree. C. which is above a softening point of a glass, to form a glass coat of a high quality. However, apart from a small roll, it requires a long period of time and an enormous amount of energy to entirely heat a huge metallic base to a temperature of 600 to 1000.degree. C., which is commonly used for the previously mentioned industrial roll whose diameter and length sometimes amount to 200 to 300 mm and 1000 mm or more, respectively.
On the other hand, when a metallic base is kept under such a high temperature, a base made of an iron material such as SS41 may be deteriorated or a serious problem in forming a glass coating may occur in a base made of a martensitic stainless steel due to a significant change in thermal expansion coefficient.
Furthermore, when a glass coating having been formed by spraying is subjected to a high temperature for a long time (about 60 min or more depending on the components), the glass layer is recrystallized resulting in the deterioration of the glass.
When a glass material of a high temperature is directly sprayed onto the surface of a metallic base, the base surface is oxidized to form a oxide film (so-called black rust) resulting in a lowered adhesion strength of the glass coating to the metallic base. To prevent the formation of the oxide film, it has been proposed to provide, for example, an under-coat layer of stainless in above mentioned Kokai No. 4-99259 and an under-coat layer of an alloy of such as Ni--Cr in Kokai No. 64-13324.
However, when the metallic under-coat of stainless or an alloy is formed, air intervening between the surface of the metallic base and the under-coat is heated by high temperature upon spraying the glass material and may be trapped as air bubbles in the glass coating. The air bubbles thus formed have a diameter of 10-44 .mu.m at the smallest and of 66 .mu.m at the largest and appear on the surface of the glass coating after polishing as minute pin holes resulting in lowered commercial value of the product.
The adhesion between the metallic under coat layer of stainless or an alloy and the glass coating is not good. In addition, the adhesion rate of stainless or an alloy upon forming the metallic under coat layer by spraying, is about 60 to 70% and the availability of the material is not good.